1. Field of the Invention
The present invention is directed to radio communications systems, and more particularly to a method of allocating slot share to unslotted and slotted messages in a CDMA (code division multiple access) paging system.
2. Description of the Prior Art
Modern wireless systems using wideband multiple access technologies (e.g. wideband CDMA referred to as W-CDMA or CDMA2000) effectively handle large variety of services, such as analog (voice) and data services. The link between a mobile user (MU) station and a base transceiver station (BTS) in a CDMA system is a multiple radio access channel, where users share the same physical radio channel (i.e. the signals transmitted by a MU occupy the same bandwidth as the signals transmitted by other MUs in the respective cell), each MU being uniquely identified by a spreading code. The forward CDMA channel (from the BTS to the MUs) is the sum of the forward pilot channel, the forward synchronization channel, one or more forward common (overhead control signalling) channels, and a plurality of forward user traffic channels that are each modulated with a distinct channel code and are combined with a PN spreading sequence. The reverse CDMA channel (from the MU to the BTS) is the sum of one or more reverse access channels and many reverse user traffic channels that are each modulated with a unique channel code and are combined with a specific PN spreading sequence.
The forward common channel is partitioned into time slots that have specified beginning and ending times, each MU being assigned a time slot once it gained access to the system. During the idle state, the MU may monitor particular slots such as its assigned slot, and sleep the rest of the time to save battery life. This mode of operation is referred to as the slotted mode. The MU may also operate in an unslotted mode, wherein it monitors all slots; this happens when the mobile operates to access the system. As such, the forward common channel transmits both slotted and unslotted messages. For each of the slotted and unslotted messages there can be primary (new) and repeat messages (i.e. messages that were not transmitted in the previous time slot or in a previous slot cycle for slotted messages).
Frames of data must be uniformly transferred from the BTS to the MU's. Thus, a scheduler for the common channels, such as paging channel (PCH) or forward common control channel (FCCCH) arranges and formats the sporadic and periodic messages into a uniform stream of data frames. The scheduler receives the messages generated by the BTS (base transceiver station) controller, arranges them according to a priority scheme and uniformly transfers them to a modulator for transmission. The overhead (OVH) messages have a top priority and as such must be scheduled periodically, while the slotted and unslotted messages are provided to the scheduler sporadically, as they are created. Currently, these messages are arranged into two separate groups (queues); the slotted messages have a second, compared to OVH messages, but qualified priority, while the unslotted messages have the lowest priority. The repeat messages take priority over the primary messages within each of the slotted and unslotted message groups.
With the advent of short messaging services (SMS), enhanced messaging services (EMS) and multimedia messaging services (MMS), the forward common channels (PCH or FCCCH) are expected to carry more traffic. In addition, paging traffic volume mix pattern and bursts for both slotted and unslotted messages vary over time. As the forward common channel traffic increases both in terms of messages to be scheduled and in the size of the messages, the current scheme of assigning a higher priority to the slotted message results in a high drop rate and delays for unslotted messages, and thus in difficulties for a subscriber to gaining access to the wireless system. CDMA call setup success rate and delay heavily depend on how efficiently the paging messages are scheduled, especially for unslotted messages, which are mostly comprised of call setup messages.
In order to address these issues, it has been proposed to set an indicator for the unslotted messages queue, to advise when the queue length exceeds a given threshold. However, this scheme does not provide a balance between the slotted and unslotted queue processing priorities and does not provision for how to resolve the unslotted messages queue when the threshold is surpassed. As a result, it may result in higher drop rates and delays for slotted messages when a large volume of slotted messages is injected into the system. There is also an issue regarding how to determine the threshold under varying traffic patterns. If it is too small, its use will degrade performances for slotted and overall paging messages. If it is too large, the improvement in the processing speed of the unslotted messages could be minimal.
What is needed therefore is an intelligent method of scheduling the slotted and unslotted messages transmitted over a forward common channel, which achieves a lower paging drop rate and delay.